caputils: cast g_malloc pointer.

caputils: cast g_malloc pointer.

Fix compilation on FreeBBSD:

../caputils/capture-pcap-util.c: In function 'if_info_get':
../caputils/capture-pcap-util.c:383:17: error: request for implicit conversion from 'gpointer' {aka 'void *'} to 'char *' not permitted in C++ [-Werror=c++-compat]
     description = g_malloc(descr_size);
                 ^

Change-Id: I4a3452a8b21c2bee853a50a977c418dd92a5d4b4
Reviewed-on: https://code.wireshark.org/review/34152
Reviewed-by: Guy Harris <guy@alum.mit.edu>
Petri-Dish: Guy Harris <guy@alum.mit.edu>
Tested-by: Petri Dish Buildbot